Zygo test report for my TMB 105/650 LZOS

[dannybgoode]

Thought this may be of interest to some.  Took my scope into the fine chaps at Rother Valley Optics the other day just for them to give it a quick once over as I've been paranoid about lens fungus.  They assured me it was fine which was great news and since I was there I left it to make use of their Zygo test service as well as I was interested to see how it measures.  This is an original Thomas Back built scope and is my baby and I am ultra paranoid about anything happening to her!

And here is the report - looks a really nice optic :).  They also gave the cell a good clean and then recollimated it as well.  Overall a first class service and well worth the money

And of course my scope is now ready for me to get down and actually start doing some imaging and observing....

 

Edited December 22, 2021 by dannybgoode

[John]

That does look good.

Whenever I've seen independent tests done on LZOS objectives they have been even better than the original test results that were supplied with the lenses by LZOS.

 

Edited December 22, 2021 by John

[dannybgoode]

The guys thought it looked good for sure.  Just that little peak right at the very edge ruining the P-V  .  I never had the original certificate and Markus couldn't track it down for my cell so now RVO have the Zygo thought I may as well see what the numbers are :). 

Could the test rigs be a little more accurate these days than when the optics were first tested perhaps?

[John]

3 minutes ago, dannybgoode said:

..Could the test rigs be a little more accurate these days than when the optics were first tested perhaps?

Possibly. The original certificates were to demonstrate that the objective met or exceeded the required specifications.

Do you know what wavelength of light your latest tests are done in ?. The originals are done in 532 nm which is green I believe.

[dannybgoode]

1 minute ago, John said:

Do you know what wavelength of light your latest tests are done in ?

I don't but will ask the guys...

[dannybgoode]

@John - RVO test at 632.8nm ...

[Space Hopper]

I have the same optic, although a much newer one (March 2019  No. 509)

My Russian interferometry report, done at Lytkarino  i assume, does indeed state the test was conducted at 532 nm.

I know the resulting Strehl ratio on my certificate : but i'd be fascinated to have it tested by RVO and compare the test reports side by side.

Maybe next year i'll pay them a visit (they are only 30 miles away) and get them to test it. I could also get my TEC optic tested as well there is no

certification with it.

Of course, all these figures are just figures. So many other things come into play as well.

As Dylan O'Donnell would say..........."all of this is meaningless.......and we're all going to die" ! 😉

 

 

[John]

21 minutes ago, dannybgoode said:

@John - RVO test at 632.8nm ...

That is red I think.

 

[dannybgoode]

7 minutes ago, Space Hopper said:

Of course, all these figures are just figures. So many other things come into play as well.

Absolutely however as I don't have the original report I was keen to have one just for interest really.  Given my Bortle 100000000000 skies and pretty much permanent cloud cover the figures are indeed meaningless  !  Plus it got cleaned and collimated also and the objective is now looking really good.

@John Specifically the red generated by a helium-neon laser  (I looked that up BTW, I am no laser expert).

Edited December 22, 2021 by dannybgoode

[Deadlake]

Green is a tougher test, but I'd be tempted to find out the poly Strehl of my LZOS at some point...

Edited December 22, 2021 by Deadlake

[Space Hopper]

10 minutes ago, dannybgoode said:

Absolutely however as I don't have the original report I was keen to have one just for interest really.  Given my Bortle 100000000000 skies and pretty much permanent cloud cover the figures are indeed meaningless  !  Plus it got cleaned and collimated also and the objective is now looking really good.

 

Someone else with Bortle 10 billion skies....??!!

I thought i was unique here in Derby.....😃  A 30s DSLR exposure of my back garden here in complete darkness literally turns night into day.

Good to hear your optic is back to its best : any chance of any pictures of the scope ?

[dannybgoode]

Just now, Space Hopper said:

Someone else with Bortle 10 billion skies....??!!

I thought i was unique here in Derby.....😃  A 30s DSLR exposure of my back garden here in complete darkness literally turns night into day.

Good to hear your optic is back to its best : any chance of any pictures of the scope ?

I will always indulge in pictures :).  Will get it out and take some snaps.  I am not far from Hillsborough football ground in Sheffield with a patio facing a row of houses, each with huge security lights etc so as well as just the general city light I also have localised light pollution also.  Still, I get some images and now the scope is back to its best, I have finally fully completed everything I need to get things set up quickly each time etc I am hoping to do more work.

[dannybgoode]

@Space Hopper - in the meantime have some outright objective porn.  This is my scope on the testbench...

 

[Victor Boesen]

Just seen this shared on Facebook by RVO. Definitely a very good looking lens on that test bench! Just noticed you can see the small red laser dot on the lens as well.

[Space Hopper]

It looks a beauty. 

Fascinating stuff. I bet it was interesting to watch it being done.

[dannybgoode]

1 minute ago, Space Hopper said:

It looks a beauty. 

Fascinating stuff. I bet it was interesting to watch it being done.

Couldn't watch. This is was all done in their clean room. They just took some pics to share

[John]

Most zygo testing seems to be done at that wavelength - 632.8nm. The Orion Optics test reports are done at that wavelength. I believe that it is quite complex and there is some additional cost to do a zygo test using another wavelength because the laser used has to be changed.

I think the advantage of testing in green light is that it is closer to how our eyes see. I did see a rough conversion formula to estimate what a PV reading in red light might convert to if done in green light which involved multiplying the figure by 1.2 or 1.3. I don't know if that works or not though.

 

 

[Andrew_B]

It’s impressive that the Strehl was so high in res light given that you’d expect the lens to be optimised to deliver its best performance in green light. 
 

I doubt the little peak near the edge has much effect on overall performance. There’s a reason very high end optics always have their surface accuracy specified as the RMS value rather than P-V. 

[dannybgoode]

5 minutes ago, Andrew_B said:

It’s impressive that the Strehl was so high in res light given that you’d expect the lens to be optimised to deliver its best performance in green light. 
 

I doubt the little peak near the edge has much effect on overall performance. There’s a reason very high end optics always have their surface accuracy specified as the RMS value rather than P-V. 

I found this on that very subject from Thomas himself:

---

   Strehl ratio is defined as the illumination at the center
of the Airy disk for an aberrated system expressed as a
fraction of the corresponding illumination for a perfect
system (i.e., 1.00). It is not calculated by P-V, but by the
RMS value. As my friend Valery Deryuzhin of ARIES
INSTRUMENTS Co. is fond of saying, if a drop of dew
forms on your lens, what is the P-V of the systems now?
And would you see any performance difference? No,
because very small localized errors have no affect on
overall performance.
   As we sample a few thousand discrete points over 100%
of the aperture, in a three element, six surface lens, the P-V
measures relatively low as compared to smaller sampling or
knife edge tests. It is the RMS wavefront measurement that
really defines the quality of the optic, and thus the Strehl
ratio. As{*filter*} Suiter says, an optic with a Strehl ratio of .88
to 1.0 is "excellent to perfect." Zeiss' standard for its highly
regarded APQ triplet apo objectives were .95 or better. We
agree, and a lens with a Strehl of .972 is certainly a planetary
lens.    
Thomas Back
TMB Optical

---

[John]

1 hour ago, Andrew_B said:

It’s impressive that the Strehl was so high in res light given that you’d expect the lens to be optimised to deliver its best performance in green light. 
 

I doubt the little peak near the edge has much effect on overall performance. There’s a reason very high end optics always have their surface accuracy specified as the RMS value rather than P-V. 

I think green light measurement is a little harsher than red, at least where visual observing is the intended use.

The real proof is in the star test though and how challenging "on the edge for the aperture" targets are handled, IMHO.

 

 

Edited December 22, 2021 by John

[Captain Scarlet]

Interesting. My one is not far from yours in the production run: yours is No 048, mine is 026.

[dannybgoode]

1 minute ago, Captain Magenta said:

Interesting. My one is not far from yours in the production run: yours is No 048, mine is 026.

Nice. Yes I saw the For Sale post for yours and it is an original Thomas Back CNC also with the outrageously engineered focuser. Lovely thing to use

Mine isn’t going anywhere!

[Andrew_B]

3 hours ago, John said:

I think green light measurement is a little harsher than red, at least where visual observing is the intended use.

The real proof is in the star test though and how challenging "on the edge for the aperture" targets are handled, IMHO.

 

 

Good point. Green light is obviously shorter wavelength so 1/50 lambda RMS at 532nm is therefore a more demanding standard than at 633nm. The highest specification I've seen quoted was 1/300 lambda at 193nm for the lenses used in lithography equipment which must be extraordinarily expensive to make (350mm fluorite blanks anyone?).

I get that Strehl ratio is a direct measure of the 'quality' of the image produced by the lens based on how the peak intensity of the Airy disc from a point source compares to the theoretical intensity from a similar perfect optic. What I'm not clear about is whether the RMS value is measuring the surface roughness of the lens surfaces or whether it relates to aberration of the wavefront. It strikes me that you could have a lens that's been figured and polished to an incredible level of smoothness, but it could also be the wrong shape to produce a good image.

[niallk]

My understanding is that RMS is computed from a grid of sampling points across the optic: I can't remember exactly, but I believe in OMI's test setup for example, 300 points or so are recorded, and they tested at 523nm.  That's a good sample size for computing a 1-sigma estimate... the reported RMS number.

However, it doesn't necessarily capture smoothness, due to the finite sampling grid.  As you mention, it's comparing the shape at the sample points versus the theoretical.

I'm not an expert and open to correction, but a test that is 'continuous' can show poor surface ripple eg knife edge, or also integrating scattered light.

[dannybgoode]

7 hours ago, Andrew_B said:

It strikes me that you could have a lens that's been figured and polished to an incredible level of smoothness, but it could also be the wrong shape to produce a good image

This is precisely what happened with Hubble's mirror. There is a misconception that the mirror was badly ground somehow when in fact it was essentially as perfectly smooth as they could make it given the constraints of the technology but was the wrong curve. 

Indeed it was because it was so perfectly wrong (it that makes sense!) that they could make the corrector lens.